Production of a semiconductor device involves a step of forming an electroconductive film on the surface of a wafer to form a wiring layer by photolithography, etching etc., a step of forming an interlaminar insulating film on the wiring layer, etc., and an uneven surface made of an electroconductive material such as metal and an insulating material is generated on the surface of a wafer by these steps. In recent years, processing for fine wiring and multilayer wiring is advancing for the purpose of higher integration of semiconductor integrated circuits, and accordingly techniques of planarizing an uneven surface of a wafer have become important.
As the method of planarizing an uneven surface of a wafer, a CMP method is generally used. CMP is a technique wherein while the surface of a wafer to be polished is pressed against a polishing surface of a polishing pad, the surface of the wafer is polished with an abrasive in the form of slurry having abrasive grains dispersed therein (hereinafter, referred to as slurry).
As shown in FIG. 1, a polishing apparatus used generally in CMP is provided for example with a polishing platen 2 for supporting a polishing pad 1, a supporting stand (polishing head) 5 for supporting a polished material (wafer) 4, a backing material for uniformly pressurizing a wafer, and a mechanism of feeding an abrasive. The polishing pad 1 is fitted with the polishing platen 2 for example via a double-coated tape. The polishing platen 2 and the supporting stand 5 are provided with rotating shafts 6 and 7 respectively and are arranged such that the polishing pad 1 and the polished material 4, both of which are supported by them, are opposed to each other. The supporting stand 5 is provided with a pressurizing mechanism for pushing the polished material 4 against the polishing pad 1.
When such CMP is conducted, there is a problem of judging the planarity of wafer surface. That is, the point in time when desired surface properties or planar state are reached should be detected. With respect to the thickness of an oxide film, polishing speed etc., the polishing treatment of a test wafer has been conducted by periodically treating the wafer, and after the results are confirmed, a wafer serving as a product is subjected to polishing treatment.
In this method, however, the treatment time of a test wafer and the cost for the treatment are wasteful, and a test wafer and a product wafer not subjected to processing are different in polishing results due to a loading effect unique to CMP, and accurate prediction of processing results is difficult without actual processing of the product wafer.
Accordingly, there is need in recent years for a method capable of in situ detection of the point in time when desired surface properties and thickness are attained at the time of CMP processing, in order to solve the problem described above. For such detection, various methods have been used, and from the viewpoint of measurement accuracy and spatial resolution in non-contact measurement, an optical detection means is becoming the mainstream.
The optical detection means is specifically a method of detecting the end-point of polishing by irradiating a wafer via a polishing pad through a window (light-transmitting region) with a light beam, and monitoring an interference signal generated by reflection of the light beam.
In such method, the end-point is determined by knowing an approximate depth of surface unevenness through monitoring of a change in the thickness of a surface layer of a wafer. When such change in thickness becomes equal to the thickness of the unevenness, the CMP process is finished. As a method of detecting the end-point of polishing by such optical means and a polishing pad used in the method, various methods and polishing pads have been proposed.
For example, a polishing pad having, as least a part thereof, a solid and uniform transparent polymer sheet passing a light of wavelengths of 190 nm to 3500 nm therethrough is disclosed (Patent Literature 1). Further, a polishing pad having a stepped transparent plug inserted therein is disclosed (Patent Literatures 2 and 3).
There is also disclosed a polishing pad having an optically transparent member that includes a water-insoluble matrix and water-soluble particles dispersed in the water-insoluble matrix and has a light transmittance of 0.1% or more at 400 to 800 nm (Patent Literatures 4 and 5). Both publications disclose that such a member is used as the end-point detection window.
In conventional windowed polishing pads, however, the window surface tends to be scratched by a dresser or a slurry and gradually reduced in light transmittance as they are used. Therefore, conventional windowed polishing pads have a problem in that when a program based on the initial light transmittance (optical reflectance) is used to detect the end-point, an error may occur in the end-point detection as the light transmittance decreases from the middle stage to the final stage of using the polishing pad.
Patent Literature 1: JP-A 11-512977
Patent Literature 2: JP-A 9-7985
Patent Literature 3: JP-A 10-83977
Patent Literature 4: JP-A 2002-324769
Patent Literature 5: JP-A 2002-324770